The present invention relates generally to tunable light sources, and more particularly to using the process of optical parametric oscillation (OPO) near degeneracy to obtain a light source with a wide and stable tuning range.
The continuing optics revolution is bringing about changes in many fields of technology. For example, fiber-optic networks employing dense wavelength division multiplexing (DWDM) are becoming increasingly pervasive as the backbone of modern communications systems. At the same time, machining devices employing lasers for precision processing, e.g., cutting, scribing and/or polishing of various materials including biological tissue are displacing traditional mechanical equipment. In still other fields, laser-based systems are being adapted for display purposes.
The above-mentioned technologies, as well as many others, require light sources with appropriate performance parameters. Specifically, there is a demand for tunable light sources, i.e., tunable lasers that can be tuned over a wide range of wavelengths. Such tunable light sources should additionally exhibit excellent spectral characteristics, e.g., clean and narrowband output as well as absence of mode hops and/or power fluctuations during the tuning process. Furthermore, suitable light sources need to be simple in construction, versatile, and economical.
Such tunable laser sources are desired, for instance, in swept wavelength testing of passive and active telecommunication components. Testing a component can include, for example, measuring transmission, reflection or loss for any combination of the component""s ports as a function of wavelength. Swept wavelength testing requires a very wide tuning range and/or a narrow test beam spectrum. In some cases a tuning range of 250 nm with a 0.1 to 10 pm test signal bandwidth is required. In addition to the swept wavelength approach, optical component testing can also be performed by a step-and-measure approach, by measurements at discrete wavelengths, or other variants of combining the tuning properties of the laser with measurements of relevant data. In this document, the term swept wavelength testing is intended to include these variants. Tunable laser sources are also employed in multi-channel coherent communication systems, spectroscopic measurements, and optical amplifier characterizations.
The prior art teaches the use of extended (or external) cavity diode lasers (ECDLs) to provide tunable laser sources for swept wavelength testing in telecommunications and other applications. A detailed description of external cavities is well documented in the art, for example, in xe2x80x9cSpectrally Narrow Pulsed Dye Laser without Beam Expanderxe2x80x9d by Littman et al., Applied Optics, Vol. 17, No. 14, pp. 2224-2227, Jul. 15, 1978; xe2x80x9cNovel geometry for single-mode scanning of tunable lasersxe2x80x9d by Littman et al., Optics Letters, Vol. 6, No. 3, pp. 117-118; xe2x80x9cExternal-Cavity diode laser using a grazing-incidence diffraction gratingxe2x80x9d by Harvey et al., Optics Letters, Vol. 16, No. 12, pp. 910-912; and xe2x80x9cWidely Tunable External Cavity Diode Lasersxe2x80x9d by Day et al., SPIE, Vol. 2378, pp. 35-41.
In a tunable ECDL the wavelength range is determined by the gain bandwidth of the lasing medium while wavelength selection and tuning functions are external to the gain element. These functions are typically accomplished by adjusting a total optical length L of the external cavity and its spectral response or passband. A diffraction grating and a movable mirror can be used for these purposes. The number of nodal points of the standing wave in the laser cavity is proportional to L/xcex, where xcex is the operating wavelength and L is the total optical length of the laser cavity (primarily provided by the length Lext of the external cavity) Therefore, if the wavelength tuning takes place while L is maintained constant, the number of nodal points in the laser cavity changes discontinuously. That is, the wavelength cannot be continuously varied, but rather, it leaps in discrete stepsxe2x80x94termed as mode-hops. As a result, it is often difficult to tune in a desired wavelength, and there may also be substantial fluctuations in the output power of the laser.
The prior art teaches to mitigate or avoid mode-hops by varying the length L of the laser cavity as wavelength tuning is taking place. Coordinating the wavelength tuning and the cavity-length change in ECDLs has been a rather arduous and expensive undertaking. Documentation of further efforts to prevent mode-hops and provide more continuous tuning are found in U.S. Pat. Nos. 5,172,390, 5,319,668, 5,347,527, 5,491,714, 5,493,575, 5,594,744, 5,862,162, 5,867,512, 6,026,100, 6,038,239, and 6,115,401.
Diode lasers typically have gain bandwidths (and therefore tuning ranges) of about 1-5% of the optical wavelength, or about 30 nm if centered near 1550 nm. Some diode lasers which are optimized for broad gain bandwidth (at the expense of other properties) can have somewhat larger gain bandwidths. Therefore, external cavity diode lasers with tuning ranges of about 50-100 nm are now commercially available. However, tuning ranges approaching 250 nm are extremely difficult or impossible to achieve with a diode laser despite all the efforts documented in the prior art.
In U.S. Pat. No. 6,134,250 the inventors describe a single-mode wavelength selectable ring laser, which operates at a single wavelength selectable from any channel passband of a multiple-channel wavelength multiplex/demultiplex element (e.g., an arrayed waveguide grating router (AWGR)). A Fabry-Perot semiconductor optical amplifier (FP-SOA) is connected to AWGR to form a ring laser structure, where FP-SOA is used as an intra-cavity narrow-band mode-selecting filter to stabilize the laser oscillation to a single axial mode. As such, this ring laser system can only provide discrete tuning from one wavelength passband of the wavelength filter to another. That is, continuous tuning cannot be achieved in this system. Hence, this prior art laser system is suited for providing a wavelength-selectable laser, as opposed to a wavelength tunable laser.
Prior art also suggests turning to other types of lasers and elements to achieve a wide and stable wavelength tuning range. Unfortunately, none of the prior art systems has the desired parameters. Specifically, the gain bandwidths for the most promising of these lasers are limited, e.g., Erbium based lasers have gain bandwidths of about 30 nm to about 100 nm, SOA has a gain bandwidth of about 30 nm and ECDLs have gain bandwidths of about 100 nm. These gain bandwidths make it impossible to provide for tuning ranges up to 250 nm or more. Furthermore, these laser sources are not sufficiently simple in construction, versatile, and economical. Combining a number of them, e.g., stitching together several ECDLs to cover a tuning range of 250 nm, is not a practical solution. This is because it is difficult to control the tuning behavior or achieve accurate wavelength control of combined sources. Furthermore, combined sources can not be tuned as rapidly as some applications require. Also, an implementation including a combination of multiple sources is generally more expensive relative to a single source which covers the required wavelength range.
In order to generate light in certain wavelength ranges where laser sources are not available (e.g., due to lack of lasing media generating light in those wavelength ranges at sufficient power levels) the prior art prescribes the use of nonlinear optics methods. Nonlinear optics encompass various processes by which a nonlinear optical material exhibiting a certain nonlinear susceptibility converts input light at an input wavelength to output light at an output wavelength in the difficult to access wavelength range. Some well-known nonlinear processes involving light at two or more wavelengths (e.g., three-wave mixing and four-wave mixing) include second harmonic and higher harmonic generation, difference frequency generation, sum frequency generation and optical parametric generation. The fundamentals of nonlinear optical processes are discussed extensively in literature and the reader is referred to Amnon Yariv, Quantum Electronics, 2nd edition, Wiley Press, 1967 for general information.
Specific methods and devices using nonlinear wavelength conversion to produce light sources are also taught by the prior art. For example, M. H. Chou et al., xe2x80x9c1.5-xcexcm-band wavelength conversion based on difference-frequency generation in LiNbO3 waveguides with integrated coupling structuresxe2x80x9d, Optics Letters, Vol. 23, No. 13, Jul. 1, 1998 teach optical frequency mixing in the 1.5 xcexcm wavelength band for telecommunication purposes. Additional information related to nonlinear wavelength conversion for communications applications can be found in I. Brenner et al., xe2x80x9cCascaded "khgr"(2) wavelength converter in LiNbO3 waveguides with counter-propagating beamsxe2x80x9d, Electronics Letters, Vol. 35, No. 14, Jul. 8, 1999; and M. H. Chou et al., xe2x80x9cStability and bandwidth enhancement of difference frequency generation (DFG)-based wavelength conversion by pump detuningxe2x80x9d, Electronics Letters, Vol. 36., No. 12, Jun. 10, 1999.
The output light from nonlinear wavelength converters can be tuned over a certain wavelength range. In general, control of the wavelengths of the mixing or interacting light beams can be used to adjust the output wavelength. When the nonlinear conversion process takes place in materials specially engineered to achieve high nonlinear conversion efficiencies, e.g., materials using quasi-phase-matching (QPM) gratings in in-diffused waveguides, control over certain grating parameters (i.e., the phasematching condition) can be employed to achieve output wavelength tuning. For general information on this subject the reader is referred to Michael L. Bortz""s Doctoral Dissertation entitled xe2x80x9cQuasi-Phasematched Optical Frequency Conversion in Lithium Niobate Waveguidesxe2x80x9d, Stanford University, 1995 as well as M. L. Bortz et al., xe2x80x9cIncreased Acceptance Bandwidth for Quasiphasematched Second Harmonic Generation in LiNbO3 Waveguidesxe2x80x9d, Electronics Letters, Vol. 30, Jan. 6, 1994, pp. 34-5. Additional information on devices using QPM gratings for nonlinear conversion in found in U.S. Pat. No. 5,875,053. The processes used to make QPM gratins are described in U.S. Pat. Nos. 5,800,767 and 6,013,221, and waveguides with QPM gratings employed for nonlinear optical processes are described in U.S. Pat. No. 5,838,720.
Some specific high power pumped mid-IR wavelength systems using non-linear frequency mixing to obtain tunable light sources are taught by Sanders et al. in U.S. Pat. No. 5,912,910. In particular, the inventors teach the use of a narrowly tunable difference frequency generation and widely tunable optical parametric oscillation for generating output light in the desired mid-IR wavelength range. The phasematching bandwidth (e.g., of the QPM grating) is used for output bandwidth control for the OPO case. Unfortunately, due to their construction and the use of phasematching bandwidth for output linewidth control the OPO light sources of Sanders et al. still exhibit tuning instability due to mode-hopping as well as large output wavelength linewidths.
The use of optical parametric oscillation for producing a continuously tunable, short pulse and high repetition rate light source is also taught by Kent Burr et al., xe2x80x9cHigh-repetition-rate femtosecond optical parametric oscillator based on periodically poled lithium niobatexe2x80x9d, Applied Physics Letters, Vol. 70, 1997, pg. 3343. The tuning bandwidth for the idler beam in this OPO extends from 1.68 xcexcm to 2.72 xcexcm and for the signal beam from 1.12 xcexcm to 1.50 xcexcm. Tuning is achieved by either temperature control of the nonlinear optical element within which OPO takes place, or tuning the wavelength of the pump beam driving the optical parametric oscillation or by adjusting the length of the cavity in which the nonlinear optical element was placed. Although low threshold for generation of output light in the form of the idler beam was achieved, these methods of controlling the OPO process do not yield a sufficiently stable and continuously tunable narrow linewidth output light desired. Furthermore, the process tends to set up double resonance (of both the idler beam and the signal beam) within the cavity when the signal and idler wavelengths are near 1560 nm. Above that, the system taught is large and bulky. Finally, the use of OPO for a tunable source is again addressed by Mark A. Arbore et al. in xe2x80x9cSingly resonant optical parametric oscillation in periodically poled lithium niobate waveguidesxe2x80x9d, Optics Letters, Vol. 22, No. 3, Feb. 1, 1997. In this case the resonant cavity is singly resonant (only at the signal wavelength) and the output wavelength (signal or idler wavelength) is efficiently generated and tuned over several hundreds of nanometers in bandwidth. The OPO is performed close to degeneracy at which the wavelengths of the signal and idler beams are equal, and the pump has half the wavelength of the signal or idler beam. Although this teaching goes far in providing a widely tunable and fairly stable light source using OPO, its output still suffers from instability and insufficiently narrow output linewidth. In fact, the output linewidth was about 4 nm, about 1,000 times too large for any practical application to swept wavelength testing, and the axial mode spacing was 2.6 GHz, causing mode-hops.
In view of the foregoing, there is still an unfilled need for a stable, simple and tunable light source having a wide tuning range, preferably over 250 nm, and a narrow output linewidth. Specifically, there is a need for an efficient, economical and widely tunable light source which can be used for practical applications in various fields of optics.
It is therefore a primary object of the present invention to provide a tunable light source which has a wide tuning range, preferably in excess of 250 nm, over which the output wavelength is stable and can be continuously tuned.
It is a further object of the invention to take advantage of the nonlinear process of optical parametric oscillation to obtain the tunable light source.
Furthermore, it is also an object of the invention to ensure that the tunable source is easy to make and control, as well as economical and well-suited for practical applications.
These and other objects and advantages of the invention will become apparent upon further reading of the specification.
The objects and advantages are achieved by a tunable light source equipped with an optical parametric amplifier (OPA) placed in a cavity for performing an optical parametric oscillation (OPO) involving a signal beam and an idler beam. The optical parametric oscillation is driven by a pump beam at a pump frequency provided to the OPA from a pump arrangement. The pump frequency is within a certain range such that the OPO is driven near degeneracy. In other words, the pump frequency is chosen such that the frequencies of the signal and idler beams are close (degeneracy being defined as the point at which these two frequencies are equal). The pump frequency tuning range expressed in terms of a wavelength tuning range is about 1.5 nm around degeneracy. The tunable source has an adjustment mechanism for adjusting the pump frequency within this wavelength tuning range and to thereby select a gain spectrum of the OPO. This gain spectrum is represented by the wavelength ranges over which the idler and signal beams experience gain. Additionally, the tunable light source has a spectral control mechanism for setting a resonant frequency of the cavity within the gain spectrum.
Conveniently, the spectral control mechanism is a narrowband tuner with its passband set or centered at the resonant frequency. The narrowband tuner can be a diffraction grating filter, a tunable fiber Bragg grating, dielectric coated mirrors, dielectric coated filters or an etalon filter. In addition to serving the primary function of selecting a particular resonant frequency within the gain spectrum, the spectral control element is also conveniently set to reject one of the idler and signal beams. In other words, only one of the idler and signal beams within the passband set by the narrowband tuner, i.e., at the resonant frequency is supported inside the cavity.
In the same or in another embodiment the cavity is a multiple axial mode cavity such that it supports a number of axial modes at the resonant frequency. The cavity can be a standing-wave type cavity or a ring cavity. Preferably, the cavity includes an optical fiber and is longer than 1 meter. It is also preferred, that the cavity be used in conjunction with the narrowband tuner for controlling the resonant frequency within the cavity.
The pump arrangement for supplying the pump beam for driving the OPO can take on any number of forms. However, it is most convenient to obtain the pump beam by relying on the nonlinear operation of second harmonic generation (SHG) to frequency double a primary beam. Thus, the pump arrangement has a light source for producing the primary beam and a second harmonic generator for receiving and frequency doubling the primary beam to produce the pump beam. The pump arrangement can also include a suitable optical amplifier, e.g., a fiber amplifier, for amplifying the primary beam.
When a second harmonic generator is used to obtain the pump beam it is convenient that both the second harmonic generator and the optical parametric amplifier be contained in the same nonlinear optical converter. This goal can be accomplished since the same nonlinear materials can be used for both second harmonic generation and optical parametric oscillation. After second harmonic generation produces the pump beam the primary beam is no longer needed. Therefore, a wavelength filter can be positioned between the second harmonic generator and the optical parametric amplifier for filtering the primary beam. Suitable wavelength filters for this purpose include a spatial mode filter, a grating, a fiber Bragg filter, a low pass filter, a directional coupler, a dichroic dielectric mirror, a grating-assisted coupler and an absorptive filter. Alternately, the residual primary beam could be further used, for example in a resonant multiple-pass configuration. In this case, the intervening filter should be chosen to provide separation of the primary beam with low loss.
In one embodiment the second harmonic generator has a first quasi-phase-matching (QPM) grating in the nonlinear optical converter and the optical parametric amplifier has a second quasi-phase-matching grating in the same nonlinear optical converter. Appropriate grating parameters are selected for phasematching the second harmonic generation and optical parametric amplification in the first and second QPM gratings, respectively. The first QPM grating for performing the second harmonic generation can be a grating with a uniform grating period or an aperiodic grating period. Preferably, the length and/or pattern of this first QPM grating is sufficient to enable efficient second harmonic generation over a bandwidth of at least 2 nm and preferably more than 3 nm for the primary beam. In addition, the two QPM gratings can be separated by a certain distance and an optical coupler can be disposed between the first and second QPM gratings for coupling in the signal beam and/or idler beam for the optical parametric amplification taking place in the second QPM grating. In this or another embodiment, it is advantageous that the QPM gratings be distributed in a waveguide fabricated in the nonlinear optical converter.
The tunable light source also has an output coupler for out-coupling at least one of the signal and idler beams. Depending on the operation, either the signal or the idler beam (or even both) can be used as the useful output of the tunable light source.
In one embodiment, the tunable light source is additionally equipped with a wavelength sweep control. The wavelength sweep control coordinates the adjustment of the pump frequency, which sets the gain spectrum, with the selection of the resonant frequency by the spectral control mechanism. Specifically, the sweep control coordinates a scan or sweep of the resonant frequency across a wavelength window. The wavelength window can have a bandwidth of 250 nm or more. For example, in swept wavelength testing applications the wavelength window can be 250 nm centered at approximately 1550 nm. Also, for the purposes of swept wavelength tests the passband for the resonant frequency can be set between 0.1 to 1000 pm, resulting in 0.1 to 100 pm output spectrum width. Furthermore, in some embodiments the tunable light source has a synchronizing unit connected to the pump arrangement for synchronizing the pump beam with a round-trip time of the cavity.
In a particular embodiment, the tunable light source is used in a swept wavelength system. The swept wavelength system preferably includes the wavelength sweep control for performing optical tests.
In another embodiment of the swept wavelength system the tunable light source has the nonlinear optical converter placed in the cavity for performing a nonlinear frequency conversion other than optical parametric amplification. For example, the nonlinear frequency conversion operation can be second harmonic generation, difference frequency generation or sum frequency generation. In all of these embodiments the nonlinear optical converter has a QPM grating for phase matching the nonlinear frequency conversion.
The present invention also provides for a method for tuning the light source using the OPA for obtaining a widely tunable output. Specifically, the method calls for producing the pump beam at the pump frequency and delivering the pump beam to the OPA for driving the optical parametric oscillation involving the idler and signal beams. Furthermore, the method calls for adjusting the pump frequency to select a gain spectrum for the OPO and setting the resonant frequency of the cavity within this gain spectrum. The OPO is then driven near degeneracy by the pump beam. The wavelength tuning range for the pump beam is approximately 1.5 nm around degeneracy.
In one embodiment, the spectrum control is achieved by establishing a passband for at least one of the idler and signal beams. In some embodiments the passband is set between 0.1 pm and 1000 pm. In the embodiments where the passband is obtained with the aid of a narrowband tuner, the tuner can be additionally used to remove one of the idler and signal beams. This removal can be based on which beam is the useful output of the tunable light source. The narrowband tuner can also be used to remove one or both of the primary beam or pump beam.
The pump beam can be delivered to the OPA in several formats. Specifically, the pump beam can be a continuous-wave beam or a pulsed beam. For example, in the event of a pulsed beam, the beam can have a duty cycle (on time) ranging from 1% to 50%. Duty cycle is defined as the pulse duration divided by the interpulse time. Of course, other duty cycle ranges can also be used, although they may result in widening of the resonant frequency bandwidth, slow tuning, low pulse frequency or all of these. In a preferred embodiment of the method, the pump beam is pulsed and synchronized with a round-trip time of the cavity. For example, the pulse repetition time of the pump beam can be synchronized to equal the cavity round-trip time, an integral number of round trip times or an integral fraction of a round-trip time. The pulse repetition time can also be adjusted to be many times longer than the round-trip time, e.g., to obtain quasi-continuous-wave operation of the light source. The pulse repetition time and pulse length can also be adjusted to provide a quasi-continuous-wave output with regard to the system using the source. This can be done by making the pulse repetition rate high relative to the frequency sensitivity of the system, or by making the pulse long relative to the response time of the system. The former is commonly referred to as xe2x80x9cquasi-cwxe2x80x9d, while the latter is commonly referred to as xe2x80x9cquasi-staticxe2x80x9d, and the former is preferred.
In operating the tunable light source the point of degeneracy is avoided. Specifically, the tunable light source is preferably operated near degeneracy but within a certain offset from degeneracy itself. Specifically, operation in a region where the separation between signal beam and idler beam is comparable to or less than the passband of the spectral control mechanism is avoided. Thus, for example, the offset can range from 1 to 1000 pm and preferably from 50 pm to 500 pm.
As will be apparent to a person skilled in the art, the invention admits of a large number of embodiments and versions. The below detailed description and drawings serve to further elucidate the principles of the invention and some of its embodiments.